1. Field of the Invention
The present invention relates to a dielectric rod antenna and, more particularly, to a portable dielectric rod antenna which is capable of receiving broadcast signals from a satellite.
2. Description of the Related Art
A conventional dielectric rod antenna has a dielectric rod made of a dielectric material and a waveguide which receives the dielectric rod so as to excite the rod. This type of antenna is used, for example, as a primary radiator of a parabolic antenna.
FIG. 8 schematically shows the general appearance of a known dielectric rod antenna. This dielectric rod antenna 10 has a dielectric rod 1, an exciting waveguide 2 receiving one end of the dielectric rod 1, a converter 3 attached to the waveguide 2 and a connector 4 provided on the converter 3. The dielectric rod 1 is made of a material which excels in its mechanical properties and which exhibits low dielectric losses, such as polypropylene, polystyrene, TPX, Teflon or the like.
In order for the dielectric antenna 10 to have a large enough gain to enable reception of broadcast signals from a satellite, the length of the dielectric rod 1 and the diameter of the cross-section perpendicular to the longitudinal axis of the dielectric rod 1 are given predetermined suitable values. For instance, when the length and the cross-sectional diameter of the dielectric rod 1 are 50 cm and 9 mm, respectively, the dielectric rod antenna 10 exhibits a gain of 23 dBi at a frequency of 12 GHz, thus clearing the minimum level (about 18 dBi or higher) of the antenna gain required for transmission in the microwave band between 10 GHz and 15 GHz.
Determination of the length and diameter of the dielectric rod 1 on the basis of the antenna gain alone, however, involves risks, such as the risk of reduction of the gain due to deflection or warp of the rod during the forming of the rod, reduction in its mechanical strength, and so forth. In addition, the portability of the dielectric rod antenna 10 may be impaired when the dielectric rod 1 is too long.